Large quantities of glucose-containing syrups are manufactured by the enzymatic hydrolysis of corn starch. This is generally carried out in two stages. In the first step, the starch is liquefied by treatment with an alpha-amylase enzyme at a pH between 6 and 7. The liquefied starch is then saccharified by means of a glucoamylase enzyme operating at a pH between 4 and 4.5. At present, the commercially-available glucoamylase enzymes used for the saccharification of starch are derived from microorganism of the genera Rhizopus and Aspergillus. These glucoamylases do not exhibit thermostability, particularly in solutions above pH 5.
The glucoamylases in current use are produced by aerobic microorganisms, i.e., those that require oxygen for growth. There are a few reports of glucoamylase being produced by anaerobic organisms. Hockenhull, et al., Biochem. J., 39, 102-106 (1945), found that the anaerobe, Clostridium acetobutylicum, produced a glucoamylase. This enzyme displayed a pH optimum at 4.5 Later Ensley, et al., J. Gen. Appl. Microbiol., 21, 51-59 (1975), studied the production of this enzyme and found that it was induced by the presence of glucose in the culture medium.
It would be desirable to hydrolyze starch by conducting the liquefaction and saccharification steps simultaneously in the same reaction mixture. This could be accomplished if a glucoamylase were available that would saccharify the liquefied starch at pH values between 6 and 7 where alpha-amylase is active. In addition, the glucoamylase would have to be sufficiently thermostable at this pH to permit the saccharification reaction to be carried out at a temperature where the reaction rate is fast enough to be useful.
We have now discovered a glucoamylase meeting these requirements that is produced by an anaerobic fermentation reaction.